Home  Key Stage 3  GCSE  ALevel  Enrichment 
: 
A Caesar shift substitution cipher is a simple cipher where every letter of the alphabet is substituted by another. To start with, each letter of the alphabet is replaced by a number.
Then we apply a shift, say 3, to the entire list by adding 3 to each number in turn. So,
Finally the number is replaced with the letter that corresponds to the letter from the initial list.
You will notice that with the letters X, Y and Z we have a problem here because no letters are assigned to 26, 27 and 28. This problem is resolved by using modular arithmetic, in this case we are using mod 26. What we do is subtract 26 from any number above 25 so our final number is between 0 and 25.
The following table shows the complete alphabet after a Caesar shift of three places. Click on the buttons below to modify the table for any shift. Caesar Shift Table Further Information Creating a Caesar shift cipher
Deciphering a Caesar shift cipher
